1. Field of the Invention
The present invention relates to semiconductor wafer cleaning and, more particularly, to techniques for more efficiently applying cleaning fluids over a wafer and improving wafer cleaning throughput.
2. Description of the Related Art
In the semiconductor chip fabrication process, it is well-known that there is a need to clean a wafer where a fabrication operation has been performed that leaves unwanted residuals on the surface of the wafer. Examples of such a fabrication operation include plasma etching (e.g., tungsten etch back (WEB)) and chemical mechanical polishing (CMP). If left on the surface of the wafer for subsequent fabrication operations, the unwanted residual material and particles may cause, among other things, defects such as scratches on the wafer surface and inappropriate interactions between metallization features. In some cases, such defects may cause devices on the wafer to become inoperable. In order to avoid the undue costs of discarding wafers having inoperable devices, it is therefore necessary to clean the wafer adequately yet efficiently after fabrication operations that leave unwanted residue on the surface of the wafer.
FIG. 1A shows a high level schematic diagram of a wafer cleaning system 50. The cleaning system 50 typically includes a load station 10 where a plurality of wafers in a cassette 14 may be inserted for cleaning through the system. Once the wafers are inserted into the load station 10, a wafer 12 may be taken from the cassette 14 and moved into a brush box one 16a, where the wafer 12 is scrubbed with selected chemicals and water (e.g., de-ionized (DI) water). The wafer 12 is then moved to a brush box two 16b. After the wafer has been scrubbed in the brush boxes 16, the wafer is moved into a spin, rinse, and dry (SRD) station 20 where DI water is sprayed onto the surface of the wafer and spun to dry. During the rinsing operation in the SRD station, the wafer rotates at about 100 rotations per minute or more. After the wafer has been placed through the SRD station 20, the wafer is moved to an unload station 22.
FIG. 1B shows a simplified view of a cleaning process performed in brush box one 16a. In brush box one 16a, the wafer 12 is inserted between a top brush 30a and a bottom brush 30b. The wafer 12 is capable of being rotated to enable the rotating brushes 30a and 30b to adequately clean the entire top and bottom surfaces of the wafer. In certain circumstances, the bottom surface of the wafer is required to be cleaned as well because contaminants from the bottom may migrate to the top surface 12a. Although both the top surface 12a and the bottom surface of the wafer are scrubbed with the brushes 30, the top surface 12a that is scrubbed with the top brush 30a is the primary surface targeted for cleaning, since the top surface 12a is where the integrated circuit devices are being fabricated.
After typical CMP operations, a wafer is placed into the cleaning station 50. In brush box one 16a, the top brush 30a and the bottom brush 30b are preferably concentrated with a cleaning chemical, which is received from a source 32. Once scrubbing is performed with the chemicals, it is generally desired to have the wafer surface 12a cleaned with water. The water cleaning is carried out such that substantially all of the chemicals used during the scrubbing are removed from the surface of the wafer 12a. In the prior art, the standard process is to pass the water through the brush (TTB).
However, because the chemical scrubbing was just completed, the brushes will be highly saturated with the cleaning chemicals. Consequently, in order to properly clean the surfaces of the wafer with water, the brushes are typically flushed with large amounts of water in an effort to remove the chemicals from the brushes and from over the wafer surfaces. Unfortunately, although the brushes are flushed with a large amount of water, a lower concentration of the cleaning chemicals remains in the brushes themselves and on the wafer surfaces. Accordingly, such a cleaning process is noticeably flawed because some chemicals used in the cleaning operation itself may remain on the wafer when the wafer is moved to the next brush box.
In some cases, the remaining chemicals can have the disadvantageous effect of causing unwanted reactions with the cleaning chemicals applied in the next brush box, and in other cases, some cleaning chemicals may remain on the wafer surface when the wafer is moved to the SRD station 20. Unwanted reactions can also have the downside of generating or introducing particulates. Furthermore, if hydrofluoric (HF) acid is used in the cleaning system 50, it is very important that substantially all of the HF be removed before the wafer is introduced to the SRD station 20. In situations where some HF remains on the wafer surfaces, the HF can have the destructive effect of eating away at the interior mechanical parts of the SRD station 20.
Assuming that the scrubbing is complete for a given wafer in brush box one 16a, and that the wafer is moved to a next station, another wafer will be introduced into brush box one 16a from the load station 10. Before the new wafer can be cleaned with the cleaning chemicals, a time must pass while the brushes 30 are brought up to the appropriate chemical concentration. This replenishing of the chemicals is necessary because during the cleaning of the prior wafer the brushes were flushed with water to remove the chemicals and perform the DI water cleaning. After some time passes, the brushes will once again be ready to be applied to the wafer so that the chemical cleaning can be performed with the brushes.
It should be apparent that the aforementioned cleaning technique is unduly inefficient. Such a cleaning process has the downside of taking more time to load the brushes with chemicals to the desired chemical concentration, flush the chemicals from the brushes to perform the water cleaning, and then re-loading the brushes with chemicals again. Not only is the process inefficient, this process can be unsafe, in that unwanted chemical reactions can occur, particulate generation can be promoted, and the mechanical components of the cleaning station 50 can thereby be placed at risk of degradation.
In view of the foregoing, there is a need for a cleaning process that avoids the problems of the prior art by improving cleaning fluid application techniques and increasing wafer cleaning throughput.
Broadly speaking, the present invention fills these needs by providing an improved method for cleaning a semiconductor wafer. The method implements a technique for maintaining the chemical concentration in the brushes at a substantially constant level throughout the wafer cleaning process. It should be appreciated that the present invention can be implemented in numerous ways, including as a process, an apparatus, a system, a device or a method. Several inventive embodiments of the present invention are described below.
In one embodiment, a method is disclosed for cleaning a surface of a wafer. The surface of the wafer is generally scrubbed with a cleaning brush that applies a chemical solution to the surface of the wafer. In this embodiment, the cleaning brush implements a through the brush (TTB) technique to apply the chemicals. The scrubbing is generally performed in a brush box, with a top cleaning brush and a bottom cleaning brush. The top cleaning brush may then be removed from contact with the top surface of the wafer. The flow of chemicals through the top brush is preferably stopped, and the chemical concentration in the top brush is preferably maintained at substantially the same concentration that was in the brush during the scrubbing operation. Next, a flow of water (preferably de-ionized water) is delivered to the surface of the wafer. The delivery of water is preferably configured to substantially remove the chemical solution from the surface of the wafer before proceeding to a next cleaning operation.
In another embodiment, a system for cleaning a semiconductor wafer is disclosed. The system includes a brush box, which has a top brush and a bottom brush for scrubbing the top surface and the bottom surface of the wafer, respectively. The brushes are configured to implement a chemical cleaning solution for the scrubbing operation. The top brush is configured to be raised from the top surface, as the wafer sits over the bottom brush and rotates against rollers. The system also contains at least one top nozzle for applying a flow of water (preferably de-ionized water) over the top surface of the wafer. The flow of water that is applied by the top nozzles is configured to remove substantially all of the chemical cleaning solution. The system may also contain at least one bottom nozzle for applying the flow of water to the bottom surface of the semiconductor wafer.
In yet another embodiment, an apparatus for cleaning a semiconductor wafer is disclosed. The apparatus contains a brush box, which includes a top brush and a bottom brush for scrubbing a top surface and a bottom surface of the wafer, respectively. The brushes may be configured to implement a chemical cleaning solution for the scrubbing operation. The wafer is configured to be held and to rotate by a set of rollers, without contacting the top and bottom brushes. The apparatus also contains at least one top nozzle for applying a flow of water over the top surface of the semiconductor wafer. The flow of water applied by the top nozzles is configured to remove substantially all of the chemical cleaning solution. The system may also contain at least one bottom nozzle for applying the flow of water to the bottom surface of the semiconductor wafer.
Advantageously, by implementing a method for maintaining the concentration in the cleaning brushes at a substantially constant level, the efficiency of the wafer cleaning process is substantially improved. The cleaning process of the present invention eliminates the time required to flush the chemicals from the brushes for the water cleaning. Also, chemicals do not have to be re-loaded into the brushes in order to prepare for the next wafer, thereby substantially reducing the waste of expensive chemicals. In addition to efficiency, the method also improves safety because it substantially eliminates unwanted chemical reactions and inhibits particulate formation. As a result, the mechanical components of the cleaning station are placed at a substantially lower risk of degradation.
Other aspects and advantages of the present invention will become apparent from the following detailed description, taken in conjunction with the accompanying drawings, illustrating by way of example the principles of the present invention.